Fix posix_memalign symbol not found with Android API 16 (#953)
Fix posix_memalign symbol not found with Android API 16 (#953)

Summary:
According to this https://github.com/android-ndk/ndk/issues/647,
  posix_memalign may not exist on Android API 16.
  From Android NDK r17c, the API exists for Android API 17+.
```
    #if __ANDROID_API__ >= 17
    int posix_memalign(void** __memptr, size_t __alignment, size_t __size) __INTRODUCED_IN(17);
    #endif /* __ANDROID_API__ >= 17 */
```
  Change the code to use posix_memalign only after Android API 17+.
  This would also fix issue for OSS React Native to pack latest folly and building with clang.
  See: https://github.com/facebook/react-native/issues/20302 and https://github.com/facebook/react-native/issues/20342
Pull Request resolved: https://github.com/facebook/folly/pull/953

Reviewed By: yfeldblum

Differential Revision: D10469757

Pulled By: Orvid

fbshipit-source-id: c63838f3f6e723ef3de77187f39597a4063043db

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly